SUPPORT PROP

Abstract: The support prop consists of an outer tube (12) with an open flared mouth (14) into which a plunger (16) projects. A deforming ring (18) in use engages the plunger and is initially located within the flared mouth of the outer tube. A chamber (26) is provided within the prop into which fluid under pressure can be introduced through an inlet (28) to the chamber to locate the prop between two surfaces by displacing the plunger relative to the outer tube. A seal (24) is located within the chamber to prevent the fluid from escaping between the plunger and the bore of the outer tube.

Description: SUPPORT PROP FIELD OF INVENTION This invention relates to a support prop of the kind which involves a tube and a plunger. PRIOR ART Yieldable support props are described in South African patent 90/1985. These support props each consist of an outer tube vith a flared mouth which accommodates a metal ring surrounding a plunger. The ring has serrations on its bore which engage the plunger. As a yielding load is applied to the prop, the plunger and the ring move downwardly into the outer tube whilst the outer tube is deformed by the ring. In order to pre-load such a prior art support prop, a set of hydraulic jacks are positioned between the ring and a clamp removably clamped to the plunger. The jacks are connected to a portable hydraulic pump to cause them to extend and thus to set the prop in place at a predetermined load. A disadvantage associated with this prior art support prop is that in practice difficulty is encountered in pre-loading the prop in the manner described. In particular during pre-loading, relative movement between the outer tube and the plunger tends to occur resulting in a loss of the pre-load. A desirable characteristic of a yieldable prop is that it should quickly reach a point at which it will yield at a substantially constant load. Furthermore the load required to cause the prop to yield must be capable of being predicted with a degree of certainty. If a yieldable prop does not yield at a substantially constant load or if the prop yields at unpredictable loads, the consequences can be catastrophic and loss of life and production may result. The prior art yieldable props have suffered from both these disadvantages. OBJECTS OF THE INVENTION It is an object of this invention to provide a support prop which can be preloaded in a manner which at least reduces the disadvantages associated with the prior art. It is a further object of this invention to provide a support prop which will yield with at least a degree of certainty. SUMMARY OF THE INVENTION According to the invention a support prop includes: an outer tube with an open flared mouth; a plunger which projects into the outer tube through its flared mouth; deforming means which in use engages the plunger and is initially located within the flared mouth of the outer tube; a chamber within the prop; and an inlet for introducing fluid under pressure to the chamber to locate the prop between two surfaces by displacing the plunger relative to the outer tube. The support prop may include a seal to prevent fluid from escaping between the plunger and the bore of the outer tube. The seal is preferably a floating cup seal. An end cap may be secured to the end of the plunger located within the outer tube. A bearing portion which bears against the bore of the outer tube may be provided to locate the plunger within the outer tube to limit axial misalignment of the plunger relative to the outer tube. The bearing portion may be provided on the end cap of the plunger. The deforming means may have an inner surface with gripping formations which are harder than the plunger so that the gripping formations can bite into the plunger to engage the plunger. The deforming means may have an outer surface with spaced recesses and outer tube contact zones located between the spaced recesses. The outer tube contact zones may taper from their upper ends towards their lower ends. The bottom end of the deforming means is preferably radiused. At least part of the bore of the outer metal tube may be coated with friction reducing means . In addition or alternatively the outer surface of the deformmg means may be coated with friction reducing means. The friction reducing means may comprise any one of oil, polytetrafluoroethylene, molybdenum disulphide or graphite. A pedestal is preferably provided inside the outer tube on which pedestal the plunger can seat when the plunger is fully retracted. The plunger is preferably a tube. An aperture may be provided in the sidewall of the outer tube through which aperture fluid can escape from the chamber if the plunger is displaced beyond the aperture in the direction of the flared mouth of the outer tube. At least one marking may be provided on the outer surface of the plunger to indicate over extension or impending over extension of the plunger. The inlet may be a male or a female coupling. The coupling may be located partly or wholly within the prop. Protecting means may be provided for protecting the inlet. In one form of the invention the protecting means comprises a handle on the prop which handle is preferably located adjacent to the inlet. According to another aspect of the invention a support prop includes: an outer tube with an open flared mouth; a plunger which projects into the outer tube through its flared mouth; deforming means which in use engages the plunger and is initially located within the flared mouth of the outer tube; and friction reducing means applied to at least part of the bore of the outer tube and/or the outer surface of the deforming means. According to yet another aspect of the invention a method of installing a support prop having an outer tube with an open flared mouth for deforming means and a plunger which projects into the outer tube through its flared mouth includes the steps of introducing fluid under pressure into a chamber within the support prop to locate the support prop between two surfaces by displacing the plunger relative to the outer tube, and thereafter allowing the fluid to drain from the chamber once the deforming means has engaged the plunger. DESCRIPTION OF THE DRAWINGS The invention will now be described by way of a non-limiting example with reference to the accompanying drawings in which: figure 1 is a cross-sectional side view of a support prop according to the invention in its fully retracted position; figure 2 is a cross-sectional side view of the support prop in its fully extended position; figure 3 is a cross-sectional side view of the support prop in an over extended position; figure 4 is a cross-sectional plan view on line IV - IV of figure 3; figure 5 is an enlarged cross-sectional side view of the inlet with an exploded view of a spigot and locking member; figure 6 is the same view as figure 5 but with the spigot inserted and locked to the inlet; figure 7 is a cross-sectional side view of a cup seal; figure 8 is a perspective view of an end cap for a plunger of the support prop; figure 9 is a cross-sectional side view of the end cap of figure 8; figure 10 is an enlarged cross-sectional side view of part of the prop; figure 11 is an alternative embodiment to that shown in figure 10; figure 12 is a perspective view of a deforming ring; figure 13 is a plan view of the deforming ring; figure 14 is a cross-sectional side view on line XIV - XIV of figure 13; figure 15 is an enlarged view of the portion 16 circled and marked XV in figure 14; figures 16 and 17 are cross-sectional side views of alternative embodiments of the deforming ring; figure 18 depicts graphs of the load/yield characteristics of two prior art yieldable props; and figure 19 depicts graphs of the load/yield characteristics of two yieldable props according to the invention. - i - DETAILED DESCRIPTION OF THE DRAWINGS Referring firstly to figures 1 to 3, a support prop 10 includes a circular cylindrical, outer steel tube 12 with an open flared mouth 14. A plunger 16 in the form of a circular cylindrical steel tube projects into the outer tube 12 through its flared mouth 14. A deforming ring 18 is located within the flared mouth 14. The plunger 16 has an end cap 20 tack welded to its end located within the outer tube 12 and a domed end 22 tack welded to its opposite end. A floating cup seal 24 abuts the end cap 20. The term "floating" is used herein to refer to a seal which is not attached to the end cap 20. A pressure chamber 26 is located within the outer tube 12. The cup seal 24 defines the upper limit of the pressure chamber 26 when the plunger 16 is in its fully extended position and when the cup seal 24 abuts the end cap 20. The chamber 26 has an inlet 28. A pedestal 30 with a plastic cap 32 projects upwardly into the outer tube 12 from a domed end 34 which is a press fit in the outer tube 12. When the plunger is in its fully retracted position as shown in figure 1 , the cup seal 24 seats on the plastic cap 32 of the pedestal 30. In this position the cup seal 24 does not abut the inwardly projecting portion of the inlet 28. The cup seal 24 is thus protected from damage by this inwardly projecting portion of the inlet 28. A seal 36 with an aperture 37 through which the pedestal 30 projects is provided on the inside of the domed end 34. The seal 36 seals the junction between the outer tube 12 and the domed end 34. The domed end 34 could equally well be welded by a continuous watertight weld to the outer tube 12 in which case the need for the seal 36 would be obviated. An aperture 38 is provided in the side wall of die outer tube 12. A guard 40 in me form of a channel is provided adjacent to the aperture 38. The applicant has found that to limit angular misalignment of the plunger 16 within the outer tube 12, the plunger 16 must have a ininimum insertion depth in the outer tube 12 when the prop 10 is at its maximum extension. If the plunger 16 is extended beyond the required minimum insertion depth, die applicant believes that the probability of the prop 10 failing due to buckling will be increased. To prevent the plunger 16 from being extended beyond the required minimum insertion depth, the aperture 38 is provided in the sidewall of die outer tube 12. The aperture 38 will be uncovered when die plunger 16 is extended beyond its minimum insertion depth. Since the diameter of the bore of the aperture 38 is greater than that of the diameter of the bore of the inlet 28, all the water introduced into the pressure chamber 28 will be discharged through the aperture 38. Thus the plunger 16 cannot then be extended any further under the influence of the water. The outer surface of the plunger 16 has an orange marking 39 in the form of a ring and a red marking 41 in the form of a ring. The markings are positioned on me plunger 16 such that when the orange marking 39 is exposed, it provides a warning that the plunger 16 is about to be extended beyond its minimum insertion deptii. If the red marking 41 is exposed, me plunger 14 has been extended beyond its minimum insertion depth, and water will be discharged through the aperture 38. The force of a jet of water escaping from the aperture 38 will be dissipated against the guard 40. Two handles 42 are welded to die outer tube 12 to facilitate handling of me prop 10. Referring now to figures 5 and 6, die axis of the inlet 28 is inclined at 30° to the horizontal to facilitate access to the inlet 28. The diameter of the bore 44 of the inlet is 10mm. A spigot 46, fastened to a hose pipe 48, can be inserted into the inlet 28. The inlet 28 has a collar 50 as does the spigot 46 which has a collar 52. Once the spigot 46 is inserted into the inlet 28, the spigot 46 is releasably locked to die inlet by a locking member 54 via the collars 50 and 52. An O-ring 56 provides a seal between the spigot 46 and die inlet 28. Referring now to figure 7, the cup seal 24 has a base 58 and an outwardly flared, peripherally extending skirt 60. The skirt 60 is at least 25mm long, and preferably 40mm long, to prevent it from tilting within the outer tube. The cup seal 24 is made of natural rubber which has a Shore hardness of 70. The cup seal 58 may however be made of nitrile. Referring now to figures 8 to 10, the end cap 20 has a planar lower surface 62, an upper surface with a circular recess 64 and a circular cylindrical bearing portion 66. A continuous, upwardly extending lip 68 on the end cap 20 locates widiin the bore of the plunger 16. The plunger 16 seats on a flat face 70. A 45° chamfered surface 72 is provided to facilitate tack welding die end cap 20 to die plunger 16. The length of die circular cylindrical bearing portion 66 depends on die diameter of me bore of an outer tube 12 of die prop 10. Generally for bore diameters varying between 127mm and 219mm, the lengdi of the bearing portion 66 varies between 10mm and 20mm. As can be seen from figure 10, the bearing portion 66 bears against the bore of the outer tube 12 and dius axially aligns die plunger 16 and die outer tube 12 to limit eccentric loading of die prop 10 thereby to reduce die probability of the prop failing due to buckling. Figure 11 illustrates an alternative end cap 20.1 and cup seal 24.1. The cup seal 24.1 is not a floating cup seal since it is attached to die end cap 20.1. The flared skirt 60 of die cup seal 24 allows for varying tolerances and surface finishes of die bore of the outer tube 12 so diat the plunger 16 can telescope relatively easily within the outer tube 12. An additional advantage of die cup seal 24 is diat the same sized seal can be used where the outer diameter of the outer tube 12 is constant but where the wall thickness of the outer tube 12 is different. For example, the wall thickness of the outer tube 12 may vary between 4mm and 6mm depending on the load die prop is designed to carry, although die outer diameter of die outer tube 12 is constant. In such a case, a seal of one size could be used for the props because die flared skirt 60 allows for the different internal diameters. It will be appreciated mat die seal will be able, widiin reason, to tolerate widely varying surfaces finishes on die bore of the outer tube. The outer tube may for example be standard electric resistance welded tubing. In an alternative arrangement, a suitable seal such as a O-ring may be provided on die end cap 20 or on the leading end of die plunger 16. In such a case, if a suitable aperture is provided through the end cap 20, die entire interior of the prop may be placed under hydraulic pressure to pre-load die prop. Referring now to figures 12 to 15, a cast or machined steel deforming ring 18 has an inner surface 74 and an outer surface 76. The outer surface 76 has circumferentially spaced recesses 78 with tapered outer tube contact zones 80 located between d e recesses 78. A gap or split 82 is provided in the deforming ring 18. A continuous, common tube contact zone 84 is located below the bottom of the recesses 78. The tube contact zones 80 taper from the top 86 of the deforming ring. The common tube contact zone 84 is also tapered and its taper is simply a continuation of the taper on the tube contact zones 80. The bottom 88 of the deforming ring 18 is radiused to prevent die deforming ring from digging into me outer tube 12 as the prop 10 yields. Five axially spaced gripping formations 90 project inwardly from the inner surface 74 of die deforming ring 18. The gripping formations 90 are harder than the plunger 16 of the prop 10 so that the gripping formations 90 can dig or bite into the outer surface of the plunger 16 to engage die plunger. In the case where plunger 16 is made of mild steel, die applicant has found diat heat treating the deforming ring 18 to obtain a Rockwell C hardness of 55 is sufficient to ensure that die gripping formations 90 will dig into the plunger 16. Figure 16 and 17 illustrate alternative embodiments of die deforming ring 18. The deforming ring 18.1 in figure 16 has gripping formations 90.1, whereas me deforming ring 18.2 has gripping formations 90.2 which have a right- angled triangular cross-section. The bore of die outer tube 12 is coated with a layer of oil. The bore may be coated widi any otiier suitable lubricant such as grease or die like. The oil ensures that there is a substantially constant coefficient of friction between the deforming ring 18 and die bore of the outer tube 12 as die deforming ring 18 moves downwardly within the outer tube 12. A substantially constant coefficient of friction ensures diat the prop will yield at a substantially predictable load and that it will then yield substantially constantly. The prop 10 is preloaded by introducing water under pressure into the chamber 26 via the inlet 28. Sufficient hydraulic pressure is applied to provide die required pre-loading of the prop 10. Generally the hydraulic pressure will be between 3MPa and 15MPa. The preload or force exerted by the prop under die influence of die hydraulic pressure is dependent on die diameter of the bore of the outer tube 12. Whilst the hydraulic pressure is being applied, the deforming ring 18 is forced into the flared moutii 14 of the outer tube 12. This may be done by means of hammering against the deforming ring 18 with a suitable tool (not shown), until die deforming ring 18 engages die plunger 16 via the gripping formations 90. The hydraulic pressure is then released and die water within the chamber 26 is simply allowed to drain out dirough the inlet 28. It will be appreciated mat if the flow rate of die water entering die chamber 26 is sufficient, die seal 24 could be omitted. In such a case die prop could still be preloaded provided more water is introduced into the chamber than escapes between die plunger 16 and die bore of the outer tube 12. As die hanging wall 92 moves closer to the foot wall 94, so die plunger 16 and die deforming ring 18 move downwardly widiin the outer tube 12 as die deforming ring 18 deforms the outer tube 12 outwardly. Referring now to figures 18 and 19, die graphs of the two figures are produced by four props which were identical in all respects except that the bores of die outer steel tubes which produced die graphs of figure 19 were lubricated, whereas die bores of the outer steel tubes which produced die graphs of figure 18 were not lubricated. From the two graphs of figure 18, it can be seen that the yield points and die load bearing characteristics of the two identical unlubricated props vary over a relatively wide range. However from die two graphs of figure 19, it can be seen diat die yield points and die load bearing characteristics of the two identical lubricated props are very similar. Thus the yield points and load bearing characteristics of the lubricated props according to die invention can be predicted witii a reasonable degree of accuracy. It will be appreciated diat many modifications or variations of die invention are possible witiiout departing from the spirit or scope of the invention.

Claims: 1. A support prop including: an outer mbe with an open flared moutii; a plunger which projects into the outer tube dirough its flared mouth; deforming means which in use engages the plunger and is initially located widiin die flared moutii of the outer tube; a chamber within the prop; and an inlet for introducing fluid under pressure to die chamber to locate the prop between two surfaces by displacing die plunger relative to me outer tube. 2. The support prop of claim 1 including a seal to prevent fluid from escaping between die plunger and die bore of die outer tube. 3 The support prop of claim 2 wherein die seal is a floating cup seal. 4. The support prop of claim 1 including an end cap secured to d e end of die plunger located within the outer mbe. 5. The support prop of claim 1 including a bearing portion which bears against die bore of the outer tube to locate die plunger widiin die outer mbe to limit axial misalignment of me plunger relative to the outer tobe. 6. The support prop of claim 1 wherein the deforming means has an inner surface with gripping formations which are harder tiian the plunger so that d e gripping formations can bite into the plunger to engage die plunger. 7. The support prop of claim 1 wherein the deforming means has an outer surface witii spaced recesses and outer mbe contact zones located between the spaced recesses. 8. The support prop of claim 7 wherein die outer mbe contact zones taper from their upper ends towards tiieir lower ends. 9. The support prop of claim 1 wherein the bottom end of die deforming means is radiused. 10. The support prop of claim 1 wherein at least part of the bore of the outer mbe is coated witii friction reducing means. 11. The support prop of claim 1 wherein the outer surface of the deforming means is coated witii friction reducing means. 12. The support prop of claim 10 or of claim 11 wherein the friction reducing means comprises any one of oil, polytetrafluoroethylene, molybdenum disulphide or graphite. 13. The support prop of claim 2 wherein a pedestal is provided inside die outer mbe on which pedestal die plunger can seat when the plunger is fully retracted. 14. The support prop of claim 1 wherein the plunger is a tube. 15. The support prop of claim 1 including an aperture in die sidewall of die outer mbe through which aperture fluid can escape from the chamber if the plunger is displaced beyond the aperture in d e direction of the flared moutii of d e outer tube. 16. The support prop of claim 1 including at least one marking on die outer surface of the plunger to indicate over extension or impending over extension of the plunger. 17. A support prop including: an outer mbe with an open flared mouth; a plunger which projects into the outer tube through its flared moutii; deforming means which in use engages die plunger and is initially located widiin die flared mouth of the outer mbe; and friction reducing means applied to at least part of the bore of d e outer mbe and/or die outer surface of the deforming means. 18. A metiiod of installing a support prop having an outer mbe witii an open flared mouth and a plunger which projects into die outer tube through its flared moutii, the method including die steps of introducing fluid under pressure into a chamber within the support prop to locate die support prop between two surfaces by displacing die plunger relative to the outer mbe, and diereafter allowing the fluid to drain from the chamber once the deforming means has been engaged witii the plunger.

Inventors: BACON STUART MICHAEL, GREENWOOD ANDREW THOMAS, HILLIER ALLAN WINSTON, SPEARING ANTHONY JOHN SPENCER, TERBLANCHE IVAN, BACON, STUART, MICHAEL, GREENWOOD, ANDREW, THOMAS, HILLIER, ALLAN, WINSTON, SPEARING, ANTHONY, JOHN, SPENCER, TERBLANCHE, IVAN
Applicants: BACON STUART MICHAEL, DORBYL LTD, GREENWOOD ANDREW THOMAS, HILLIER ALLAN WINSTON, SPEARING ANTHONY JOHN SPENCER, TERBLANCHE IVAN, BACON, STUART, MICHAEL, DORBYL LIMITED, GREENWOOD, ANDREW, THOMAS, HILLIER, ALLAN, WINSTON, SPEARING, ANTHONY, JOHN, SPENCER, TERBLANCHE, IVAN
Application year: 1994
Main classification: E21D15
IPC codes: E21D15